The precise role of pulmonary hypertension as a possible factor inducing a decrease in heart rate variability is poorly known. Spectral analysis of heart rate variability (HRV) was carried out in 21 Wistar rats before and after exposure to normoxia (N = 10) or to 3 weeks of hypobaric hypoxia inducing chronic pulmonary hypertension and right ventricular hypertrophy (N= 11). Continuous ECG was recorded in conscious animal at rest. Compared to the control group, rats exposed to hypoxia had a similar heart rate but a lower overall HRV (total power, 27.9 +/- 15.2 vs. 57.6 +/- 24.7 ms2, P < 0.01). Low frequency power (0.25-0.8 Hz) and high frequency power (0.8-3 Hz) were similar in both groups suggesting that HRV was decreased in the very low frequency power (0-0.25 Hz). The effects of atropine and propranolol on heart rate and HRV were similar in rats exposed or not to hypoxia. HRV is decreased in rats with hypoxic induced pulmonary hypertension, mainly in the very low frequency band, suggesting an increase in sympathetic activity. However, this decrease is moderate and the modulation of HRV with pharmacologic autonomic blockade remains similar to that of normal rats.